Lewis,
If you and your buddy do not have one already, download the Free 500 Summary Manual from Shopsmith directly. It is at http://www.shopsmith.com/getting_started/500_Summary_Manual.pdf
They too show using the long allen wrench with a sanding disc in a bit of a different manner than I mentioned. I have attached the page illustrating that from the manual for your review.
In it they say don't use the disk if it is 1/32" off. I don't use it if it is far less than that. More like .001" Keep a good flat one handy for alignment tasks.
As JPG elaborated, the test board needs to be flat and dimensionally true.
In all reality, it doesn't make a bit of difference either way if the board is 90° or even beveled at either of the ends, since neither end is used in the measurements in the single cut. The actual cut within it is used.
It does make a difference if the board is NOT flat on the face, back and edges and of a uniform thickness before you make your cut. That’s what JPG was referring to.
Everett
As an FYI - you can make the degree symbol from your keyboard by pressing down and holding the Alt key, and while doing so entering 248 on the Numeric Key Pad (only) and then releasing the Alt key.
That's the ASCII degree symbol. So is Alt + 0176 but I prefer the shorter method. Since we work with angles frequently, that may help you more than you realize as you write documents.
